The present invention belongs to the general class of internal combustion engine exhaust silencers or mufflers that may be characterized as attempting to achieve a "cold, wet/dry" condition, as contrasted with "cold, wet" or "hot, dry" conditions, for extracting acoustic energy from exhaust gas. A "cold, wet/dry" condition is one in which a liquid coolant, typically water, first has been added to the exhaust gas of an engine, typically a marine engine, in order to reduce the temperature of the exhaust gas (the "cold, wet" stage), and then the water has been separated from the gas (the "dry" stage) in preparation for further reduction of the acoustic energy of the "dry" gas. The reduction in temperature is desirable for two reasons. First, the lower temperature reduces the acoustic velocity in the gas, that is, the speed at which sound propagates through the gas. The lower the acoustic velocity, the smaller the chamber that may be used to achieve a given reduction in acoustic energy, or noise. Alternatively, greater noise reduction can be achieved in a given space. Second, as the exhaust gas cools, it becomes denser. Thus, the dynamic pressure of the gas passing through a tube of a given size is reduced, resulting in a reduction in the pressure drop through the tube, and, consequently, a smaller "back pressure" effect. Back pressure is undesirable because it may interfere with the efficient operation of the engine or may damage it.
One undesirable attribute of cold, wet marine-exhaust silencers is that the reduction in back pressure achieved by water cooling, as just described, is offset as a consequence of the presence of water mixed with the gas. The denser net mass of the inhomogeneous water-gas mixture, as compared to a cold, wet/dry system in which the water has been removed, or as compared to a hot, dry system in which water was never added, results in an increase in back pressure. In order to avoid excessive back pressure, water-gas velocities in cold, wet exhaust systems must be held to a range of 20 to 50 feet per second (fps). This velocity restriction places requirements on the sizes of pipes, which in some cases makes the silencers larger or less effective than desirable. Moreover, whereas in a "dry" gas silencer, i.e., either a "hot, dry" or "cold, wet/dry" silencer, the "dry gas" may be conducted to a remote discharge point using a routing of both upward and downward pitched piping, such routing is often impracticable in a "wet" silencer because of an unacceptably large increase in back pressure for upward pitches and for corners. Because the appropriate discharge of exhaust gas from the vessel may be an important safety and convenience consideration, the limitation on discharge-pipe routing imposed by mixed water and gas discharge can impose a serious design problem or constraint.
In general, prior art marine-exhaust silencers have not optimally balanced the benefits of water cooling with the need to reduce back pressure while minimizing the size of the silencer. More specifically, some prior art marine-exhaust silencers attempt to operate in a "cold, wet/dry" condition but fail to achieve sufficient separation of the water from the gas. Other designs improve on such separation at the expense of larger size and reduced flexibility of configuration.
For example, U.S. Pat. No. 5,022,877 to Harbert and U.S. Pat. No. 4,019,456 to Harbert rely on gravitational effects and condensation to separate the exhaust gas from the water coolant, thus only partially achieving a "cold, wet/dry" condition. Greater separation using these means could be achieved, but at the expense of increasing the size of the silencer; ie., by providing a larger free surface of the gas-water mixture through which the gas could rise, or at the expense of increased back pressure due to elaborate flow control. U.S. Pat. No. 4,917,640 to Miles employs such an approach by providing a more complex configuration of tubular separation chambers. Another approach, disclosed in U.S. Pat. No. 5,588,888 to Maghurious, is to agitate the wet mixture of exhaust gas and water in order to atomize the water droplets in the mixture and thereby increase the absorption of acoustic energy by the water mass. This approach thus is a variation of a cold, wet design in that it relies upon reduction in the acoustic energy of the exhaust gas before it is fully separated from the water, thereby incurring the penalties associated with cold, wet systems already noted.
Accordingly, an apparatus and method are needed that overcome the drawbacks of prior art marine-engine silencing devices and methods, in particular by achieving better separation of the exhaust gas from the liquid coolant prior to further reduction of the acoustic energy of the exhaust gas.